1. Field of Invention
This invention relates in part to improvements in methods and apparatuses for dynamic information storage or retrieval, and more particularly to improvements in methods and circuitry for positioning a transducer for writing or detecting data written onto a spinning data disk, and still more particularly to improvements in circuits for driving piezo-based milli-actuator structures and methods for making same.
2. Background of Invention
Mass data storage devices include well known hard disk drives that have one or more spinning magnetic disks or platters onto which data is recorded for storage and subsequent retrieval. Hard disk drives may be used in many applications, including personal computers, set top boxes, video and television applications, audio applications, or some mix thereof. Many applications are still being developed. Applications for hard disk drives are increasing in number, and are expected further to increase in the future.
In the construction of mass data storage devices, a data transducer, or head, is generally carried by an arm that is selectively radially positionable by a servo motor. Recently, so-called milli-motors, or milli-actuators, have been considered to provide better, or more accurate, position control of the head. A milli-actuator is generally constructed with a piezo element carried by the positionable arm and to which the head is mounted. A current is selectively applied to the piezo element, which causes the piezo element to deform, thereby moving the head a small, controllable amount. This provides a fine adjustment to the position of the head. As track densities become more dense, control of the position of the head becomes more critical. Thus, piezo-based milli-actuators are becoming of increasing importance in the head positioning mechanisms.
At least discrete circuits are available for providing drive signals to milli-actuators to control the position of the head of the device or drive with which the milli-actuator is associated. Typically, the milli-actuator drive circuits operate by supplying a control voltage to the piezo element of the milli-actuator.
Although such voltage mode circuits are relatively easy to build, they have several problems. First, the deformation response of piezo elements generally is highly temperature dependent. Thus, significant temperature compensation or calibration circuitry must be provided to assure accurate head positioning over the range of expected operating temperatures of the drive. Secondly, relatively high voltages are required to operate the piezo elements, which may require relatively large circuit components, and may complicate the overall circuit design. Thirdly, piezo elements generally have strong hysteresis effects.
As a result, charge mode milli-actuator circuits have been proposed. Charge mode milli-actuator circuits typically have a capacitor in series with the piezo element, such that a charge builds up on the capacitor that is proportional to the charge on the piezo element. The change in voltage across the capacitor is measured in a given time, and the product of the measured voltage change times the capacitance of the capacitor equals the charge on the capacitor. This value can then be used to adjust the charge supplied to the piezo element. The charge mode technique is still subject to temperature variations and hysteresis effects, but these effects are substantially reduced, and, as a result, the charge mode of operation is more accurate than the voltage mode of operation. On the other hand, the piezo elements are typically large, having capacitances in the range in the thousands of picofarads. Thus, the capacitor that must be used must be proportionately large. Also, the charge used to charge the capacitor is unavailable for charging the piezo element.
One way in which at least some of these problems in the charge mode of operation have been addressed uses a mirror circuit technique in which the milli-actuator circuit provides a 1X mirror circuit connected to a sense capacitor. An nX mirror circuit mirrors the current in the 1X mirror circuit to drive the piezo element. Thus, as the charge on the capacitor changes, the nX output proportionally changes. Thus, at least the size of the sense capacitor can be reduced.
In the past, such mirror circuit type milli-actuator circuits have been provided as separate integrated circuits in which the full positive and negative piezo element drive voltages are used to power both the 1X and nX mirror circuits. However, such positive and negative supply voltage range cannot be used if the milli-actuator circuit is to be integrated with other circuitry, such as the motor servo circuitry, or the like, since the servo circuitry is generally referenced to ground, and the connections to the full rail potentials cause noise. Also, since the substrate is at ground, ordinarily the negative piezo driving voltages cannot be brought onto the chip.
What is needed, therefore, is a milli-actuator driver circuit that can operate at preestablished voltages other than the full rail voltages of the servo.
In light of the above, therefore, it is an object of the invention to provide a milli-actuator drive circuit that has selectable voltage and charge modes of operation that need not operate at the full rail potentials.
It is another object of the invention to provide a milli-actuator drive circuit that has a charge mode of operation that uses a current mirroring technique to monitor a charge on a sense capacitor and for developing current outputs having a predetermined ratio to the monitored charge for delivery to the piezo element of the milli-actuator.
It is an advantage of the invention to provide a drive circuit for a milli-actuator in a mass data storage device, or the like, that can be integrated in an integrated circuit, which reduces the load on the high voltage switching regulator and reduces the number of required external components.
These and other objects, features and advantages will become apparent to those skilled in the art from the following detailed description, when read in conjunction with the accompanying drawings and appended claims.
According to a broad aspect of the invention, an integrated circuit is provided for supplying drive signals to a piezo element of a milli-actuator device in a mass data storage device. The integrated circuit has a first circuit for receiving head position control signals and for providing a charging current to a sense capacitor in response thereto. The first circuit is powered by a voltage supply that is referenced to a substrate potential. A second circuit for mirroring a current in the first circuit at a predetermined mirror ratio to provide drive currents to the piezo element. The substrate potential may be, for example, an analog ground potential. If desired, the integrated circuit may also include a first switch connected to selectively disable the first circuit and a second switch connected to selectively provide a feedback path from the second circuit to an input of the second circuit. When the first and second switches are selectively operated, the integrated circuit is configured to operate in a voltage mode.
According to another broad aspect of the invention, a milli-actuator driver is provided for positioning a head of a hard disk drive. The driver includes first integrated circuit means for receiving head position control signals and for providing a charging current to an external sense capacitor in response thereto. The first integrated circuit means is powered by a voltage supply that is referenced to a substrate potential. Second integrated circuit means are provided for mirroring a current in the first integrated circuit means at a predetermined mirror ratio to provide drive currents to a piezo element. The substrate potential may be, for example, an analog ground potential.
According to yet another broad aspect of the invention, an integrated circuit is provided for supplying drive signals to a piezo element of a milli-actuator device in a mass data storage device to position a data head thereof. The integrated circuit includes a current mirror, which includes first and second mirror portions. The first mirror portion is configured to receive head position control signals from a head position sensing circuit. The first mirror portion is also configured to provide a first current in response to the head position control signals for connection to a capacitor. The first mirror portion is powered by a voltage supply that is referenced to a substrate potential. The second current mirror portion is configured to mirror the first current at a predetermined mirror ratio. The second current mirror is configured to provide drive currents for connection to the piezo element.
According to yet another broad aspect of the invention, a method is provided for operating a milli-actuator driver for a mass data storage device. The method includes providing a current mirror having first and second current outputs. The first and second current outputs have a ratio of 1X:nX, and the first current output is adapted to be connected to a capacitor. The second current output is adapted to be connected to a piezo element of the milli-actuator. The method also includes providing a supply voltage to at least a first portion of the current mirror that is referenced to a ground voltage.
Each channel of the driver has an operational amplifier with a 1X output driver referenced to AGND and powered off VSUPPLY. The amplifier also has two outputs driven from the VPP supply that combine with an external low side mirror driven off VMM supply to provide a 10X current output. The amplifier can support either charge mode or voltage mode operation. To support voltage mode control of the actuator, the 1X output is disabled and the 10X output driver is used in a voltage feedback loop to drive the piezo motor. When charge mode control is required for the actuator, a voltage feedback loop using only the 1X output is used to drive an external capacitor while the 10X output, drives the piezo motor. In charge mode, an OTA error amplifier can be used to provide low frequency compensation of the piezo motor voltage so that the piezo voltage slowly tends toward ground and a high pass transfer function is realized.
The milli-actuator driver works off a single 12V input supply with a plus or minus 10% tolerance. A positive LC DCxe2x80x94DC converter is used to generate the positive high voltage supply, VPP. A charge transfer circuit off the VPP converter is used to generate the negative supply and regulation of both supplies is achieved through feedback off the VPP supply.